Conveyorized microwave ovens have been used in industry for many years to cook or thaw foods and provide heat for processing objects such as rubber and foundry cores. These ovens generally operate at 915 MHz or 2450 MHz because these frequencies are within narrow frequency bands designated by government agencies for such purpose. The intensity of microwave energy permited to leak from domestic and/or industrial microwave heating systems is restricted. In the United States, for example, the Department of Health and Human Services requires that the microwave energy leakage from a domestic oven not exceed one milliwatt per square centimeter in the factory or five milliwatts per square centimeter in the home. Further, the Occupational Safety and Health Administration requires a microwave energy exposure of less than ten milliwatts per square centimeter. The International Microwave Power Institute has adopted a standard for intensity of microwave energy radiation leakage which is "less than ten milliwatts per square centimeter". Furthermore, the Federal Communication Commission has regulations regarding the amount of out-of-band radiation permissible by a microwave oven. Accordingly, systems employing the use of microwave energy for processing of materials or cooking and thawing food must include apparatus to prevent the leakage of microwave energy from the enclosure.
Many industrial microwave heating applications require that there be a continuous access aperture into the cavity so that materials may be transported through the cavity by a conveyor to achieve high throughput. The suppression of microwave energy from these apertures has presented problems which are much more complex than a batch-type microwave oven which can be sealed by use of a door.
One prior art approach to the suppression of microwave energy from a conveyorized microwave system is to position a tunnel extending from the aperture and line the tunnel with a lossy material that absorbs the microwave energy as it propagates therethrough. The food or product passes through the tunnel on a conveyorized system. One lossy material used is foamed glass but this material is fragile, dirty, and smelly and, therefore, is not compatible with food processing. Furthermore, the loss of foamed glass is relatively low so that an extremely long tunnel is needed in order to have effective leakage suppression from a relatively high power cavity.
Another lossy material used is a fluid that can be pumped around microwave transparent conduits in the tunnel so that the heat resulting from absorption can be removed to a heat exchanger thereby reducing the temperature of the tunnel. Although this approach has an advantage over foamed glass in limiting temperature requirements of the tunnel, the plastic or glass tubes are easily broken. Also, the pumps and heat exchangers such as radiators are relatively expensive. Furthermore, this approach, like the foamed glass, requires that the tunnel be relatively long to provide adequate suppression and the cross-section through the tunnel must be relatively small.
Another prior art approach to the problem is to use a plurality of thin metal flaps that hang in a lossy wall tunnel. Product passing through the tunnel on a conveyor pushes the flaps aside. When the tunnel crosssection has mutually orthogonal dimensions that are substantially greater than a free space wavelength of the microwave energy and when product pushing aside the flaps is not sufficiently lossy, the flaps do not provide an effective seal.
All of the approaches described above require the microwave energy entering the tunnel to be absorbed by some lossy material. Accordingly, each of these approaches detracts from the efficiency of the overall system because the available microwave energy must be split between the product and the tunnel. An improved approach, such as that described in U.S. Pat. No. 4,227,063, uses a plurality of conductive posts to provide an effective choke of microwave energy. Although this approach provides enhanced system efficiency and effective sealing at the fundamental microwave frequency, the bandwidth of the choke is somewhat limited such that out-of-band harmonics and other spurious radiation propagate through. In view of the Federal Communications Commission regulations, the harmonics and other spurious radiation must also be prevented from leaking from the cavity.